1. Field of the Invention
The present invention relates to an amplifier of the so-called active-core type for optical fiber telecommunication lines and to the optical fiber telecommunication lines incorporating said amplifiers.
2. Description of the Prior Art
So-called active-core amplifiers consist of a length of active-core optical fiber which will be defined in the description that follows and a source of optical pumping radiation which will be defined in the following description as well.
An active-core optical fiber is an optical fiber the core of which, made of silica glass, contains active dopants as hereinafter defined, in addition to the dopants necessary to make said core have a refractive index higher than that of the cladding, that is the radially outermost layer made of silica glass as well.
The above-mentioned active dopants are formed with substances such as rare-earths, erbium and the like for example, which, when excited by optical radiation, known as optical pumping radiation, the wavelength of which depends on the particular dopants selected, have the property of emitting optical radiation, known as emission radiation which has a different wavelength depending however on the particular dopant selected.
Another feature of the active dopants referred to hereinbefore is the fact that, once excited by the optical pumping radiation, they are capable of emitting said optical emission radiation when optical radiation of the same wavelength as the latter impinges on them.
The source of optical pumping radiation is generally a laser and in particular a laser diode capable of emitting optical radiation of the same wavelength as the one required and necessary for exciting the active dopants present in the optical fiber core having an active core as previously defined.
Active-core amplifiers for optical fiber telecommunication lines are already known in the art.
An optical fiber amplifier having an active core for optical fiber telecommunication lines comprises a source of optical pumping radiation optically connected to a dichroic coupler to which one portion of the optical fiber of the transmission or telecommunication line is optically coupled.
In turn the dichroic coupler is directly connected to the optical fiber length having an active core and this first length is in turn connected again to a second optical fiber portion of the transmission or telecommunication line.
In the above known amplifier the source of optical pumping radiation, through the dichronic coupler, sends its own radiation to the length of active-core optical fiber where it causes the excitation of the active dopants present therein.
Also sent to the length of the active-core optical fiber through the dichronic coupler, are the signals to be amplified coming from the first optical fiber portion of the telecommunication line which necessarily must have a wavelength identical to the emission wavelength of the active dopants present in the length of active-core optical fiber.
At the moment at which an optical signal enters the length of the active-core optical fiber it encounters the dopants in an excited condition due to the optical pumping radiation and, for the reason set forth before, a massive emission of optical radiation occurs which has the same wavelength as that of the signal, which brings about, as a result, the amplification of said signal.
In the amplifiers in question and previously described, there is the problem of increasing their yield, defined as the ratio between the obtainable amplification gain and the applied pumping radiation power, while at the same time achieving the benefits of amplifiers on a large scale, capable of offering sufficient reliability so as to enable them to be used in practical applications, such as for example, their easy and safe insertion in optical fiber telecommunication lines.
In the publication "Fourteenth European Conference on Optical Communication" of Sep. 11-15, 1988, on pages 25 to 28, there are the gain values of known amplifiers, which range between 0.14 and 0.31 dB/mW. For the purpose of improving the gain of optical amplifiers, set forth in the same publication are the experimental results of the gain obtained with an optical amplifier made in the laboratory in which the length of the active-core optical fiber, single-mode both to signal and pumping optical radiations, contains erbium as an active dopant. The source of optical pumping radiation used has a wavelength of 980 nm and the optical signal radiation used has a wavelength of 1536 nm.
Even if it has not been specified in the aforementioned publication the dichroic coupler adopted in the optical amplifier hereinbefore described is inevitably a dichroic coupler of the so-called microoptics type, that is a dichroic coupler in which lenses are used in order to be able to introduce the optical pumping and signal radiations into the length of active-core optical fiber. This is due to the fact that dichroic couplers of the other existing types are unable to operate in a satisfactory manner with the particular length of active-core optical fiber in question.
By adopting this known solution a yield of 2.2 dB/mW, intended as the ratio between the gain and the pumping power used, can be reached, which value is interesting. However, that amplifier has the drawback that it is not reliable for use on a large scale due to the particular dichroic coupler adopted therein.
In fact a dichroic coupler of the microoptics type is very delicate in itself and it can hardly be inserted in an optical fiber telecommunication line, which results in unreliability of the telecommunication lines which incorporate such amplifiers.